1. Technical Field
The invention relates to structural parts of nickel-chromium-iron base alloy having an austenitic texture, suitable for use in contact with an aqueous medium, at high temperature, typically of 200.degree. to 300.degree. C.
2. Prior Art
Austenitic iron-chrome-nickel alloys have been used for long in high temperature water, in particular for constructing structural members in contact with water and steam in the primary and secondary circuits of pressurized light water reactors (PWRs).
For some of these alloys, extended experience of their use is available and their properties are well known. In particular, the so called "600" alloys have been used for constructing the tubes of the steam generators of PWRs. Such tubes are in contact on one surface with the dimineralized water in liquid and vapor phase of the secondary circuit and, on the other surface, with the water which may contain lithium and boron of the primary circuit. The X750 alloys have also been very widely used for constructing the pins fixing the control cluster guide tubes to the upper core plate, which tubes belong to the upper internals of the reactors.
Table I below gives the composition of these alloys.
TABLE I __________________________________________________________________________ C S P Mn Si Ni Cr Fe Co Ti Cu Al Nb __________________________________________________________________________ 600 &lt;0.050 .ltoreq.0.015 .ltoreq.0.015 &lt;1.00 &lt;0.50 &gt;72.00 14.00- 6.00- &lt;0.10 -- &lt;0.50 -- -- 17.00 10.00 690 0.010 .ltoreq.0.015 .ltoreq.0.015 .ltoreq.0.50 .ltoreq.0.50 .gtoreq.58.00 28.00- 7.00- &lt;0.10 .ltoreq.0.50 .ltoreq.0.50 .ltoreq.0.50 0.040 31.00 10.00 X 750 .ltoreq.0.080 .ltoreq.0.010 -- .ltoreq.1.00 .ltoreq.0.50 .gtoreq.70.00 14.00- 5.00- .ltoreq.0.20 2.25 .ltoreq.0.50 0.40 0.70 17.00 9.00 2.75 1.00 1.20 __________________________________________________________________________
Through experience, techniques of constructing and welding these alloys have been perfected brushing away the uncertainties and knowledge has been acquired about their long term resistance. But it has becoma apparent that structural members of 600 and X650 alloys exhibit stress corrosion in pure water and in the primary coolant of light water reactors when they are subjected to high mechanical stresses. This phenomenon is one of those which limit the useful life of the structure members formed from these materials which are subjected to stresses in reactors.
Alloys have also been proposed, with the purpose of reducing the generalized corrosion, having a much lower nickel content than that of the above alloys and a much higher chromium content. Among these alloys may be mentioned more especially the 690 alloys, whose composition is also given in table 1, which contain 28 to 31% of chromium and whose nickel content may be as low as 58%.
These new materials have mechanical and physical properties appreciably different from the alloys already widely used; they require new methods of construction compatible therewith which remain to be developed; they give rise to undertainties in so far as their long term resistance is concerned.